The present invention relates to a polymer electrolyte fuel cell stack and is used for portable power sources, electric vehicle power sources, and domestic cogeneration systems.
The polymer electrolyte fuel cell causes a gaseous fuel, such as gaseous hydrogen, and an oxidant gas, such as the air, to be subjected to electrochemical reactions at gas diffusion electrodes, thereby generating the electricity and the heat simultaneously.
One example of the polymer electrolyte fuel cell stack is described below with reference to FIG. 10.
A pair of catalytic reaction layers 62, which are mainly composed of carbon powder with a platinum metal catalyst carried thereon, are closely attached to opposite faces of a polymer electrolyte membrane 63, which selectively transports hydrogen ions. A pair of diffusion layers 61 having both the gas permeability and the electron conductivity are further arranged on the respective outer faces of the catalytic reaction layers 62. The catalytic reaction layer 62 and the diffusion layer 61 constitute each electrode 69. The electrodes 69 and the polymer electrolyte membrane 63 are integrally formed to construct a membrane electrode assembly (hereinafter referred to as MEA) 70. Gas sealing members and gaskets are disposed around the electrodes 69 across the polymer electrolyte membrane 63, in order to prevent supplies of gaseous fuel and oxidant gas from leaking outside the fuel cell stack or from being mixed with each other. These gas sealing members and gaskets may be integrated with the MEA 70 in advance.
Referring to FIG. 15, in the fuel cell stack having the above configuration, one sealing technique arranges sealing members 127 and O rings 128 around the electrodes 69 across the polymer electrolyte membrane 63, in order to prevent the gaseous hydrogen and the air from leaking outside the fuel cell stack or from being mixed with each other. Another sealing technique arranges gaskets 129, which are composed of an appropriate resin or metal and have substantially identical thickness with that of the electrode 69, around the electrodes 69 as shown in FIG. 16. In this structure, the clearance between a separate plate 64 and the gasket 129 is sealed with an adhesive or grease.
Another recently proposed technique shown in FIG. 17 causes specific parts of the MEA 70 that require the gas sealing property, to be impregnated previously with a resin 131, which has sealing effect and subsequently solidifies. The solidified resin 131 ensures the gas sealing property between the MEA 70 and the separator plate.
A pair of conductive separator plates 64 are arranged across the MEA 70 so as to mechanically fix the MEA 70 and cause the MEA 70 to electrically connect with the adjoining MEAs 70 in series. A specific part of the separator plate 64 that is in contact with the MEA 70 has a gas flow path 65, which feeds the supply of the gaseous fuel or the oxidant gas to the surface of the electrode 69 and flows out the gas evolved by the reaction and the remaining excess gas. The gas flow path 65 may be formed independently of the separator plate 64. As shown in FIG. 10 and FIGS. 15 through 17, however, a groove formed on the surface of the separator plate 64 generally constitutes the gas flow path 65.
Most of the fuel cell stacks have a laminate structure in which a large number of unit cells are laid one upon another. A cooling plate is provided for every one or two unit cells, in order to cause the heat produced with the electric power in the course of operation of the fuel cell stack to be out of the fuel cell stack. The cooling plate is generally a thin metal plate which a heat medium, such as cooling water, flows through. This structure makes the cell temperature kept at a substantially fixed level and enables the generated thermal energy to be unitized, for example, in the form of warm water.
Another possible application makes the separator plate 64 itself function as the cooling plate. In this case, a cooling water flow path is formed on the rear face of the separator plate 64, which is included in each unit cell, to make a flow of cooling water. In this structure, O rings and gaskets are also required to seal the heat medium, such as cooling water. The O rings in the seal should be smashed completely to ensure the sufficient electric conductivity across the cooling plate.
Such a cell laminate typically requires supply inlets and exhaust outlets of the gaseous fuel and the cooling water to and from the respective unit cells, which are respectively joined to manifolds. Various arrangements and layouts of these manifolds are classified into two groups, that is, an internal manifold type and an external manifold type.
The general arrangement is the internal manifold type, in which the supply inlets and the exhaust outlets of the gaseous fuel and the cooling water are disposed inside the cell laminate. In the case where the reformed city gas is used as the gaseous fuel to drive the cells, however, the CO concentration rises in the downstream area of the flow path of the gaseous fuel. This may cause the electrode to be poisoned with CO, which results in lowering the temperature and thereby further accelerating the poisoning of the electrode. In order to relieve the deterioration of the cell performance, the external manifold type is noted as the structure that increases the length of the gas supply and exhaust unit between the manifold and each unit cell.
In either of the internal manifold type and the external manifold type, the required process lays a plurality of unit cells including the cooling units one upon another in one direction to provide a cell laminate, arranges a pair of end plates outside the cell laminate, and fixes the space between the pair of end plates with fastening rods. It is naturally desirable to urge the whole face of each unit cell as uniformly as possible. In other words, it is desirable that the substantially uniform compressive force is applied to the whole laminating faces of the cell laminate. From the viewpoint of the mechanical strength, the end plates and the tie rods are generally made of a metal material, such as stainless steel. These end plates and fastening rods are electrically insulated from the cell laminate by insulator plates, so that the electric current does not run outside through the end plates. One proposed technique for fastening a cell laminate makes the fastening rods pierce the through holes formed in the separator plates. Another proposed technique binds the whole cell laminate on its periphery in the laminating direction with metal belts.
In any of the sealing methods shown in FIGS. 15, 16, and 17, the constant compressive force is required to maintain the sufficient sealing property and ensure the small contact resistance between the electrodes and the separator plates and between the separate plates. One adopted structure inserts a coiled spring or a belleville or disc spring between the fastening rod and the end plate. The compressive force ensures the electric contact between the respective constituents of the cells including the separator plates, the electrodes, and the electrolyte membranes.
For the stable performance of the cell laminate, it is required to supply the gaseous fuel, the oxidant gas, and the cooling water evenly to the respective unit cells. The general structure adopted for that purpose increases the cross section of the manifold, which the supplies of the respective fluids flow through. The large cross section decreases the flow rate of each fluid in the manifold and reduces the effects of the pressure gradient due to the dynamic pressure of the fluid. In order to reduce the whole size and weight of the fuel cell stack, on the other hand, it is required to minimize the cross section of the manifold.
In the conventional configuration, the electricity produced in the cell laminate is collected by a pair of current collectors and output to external equipment connected to the respective terminals of the current collectors. In the case of the current collectors each having a specific extension protruded from the contour of the cell laminate, the specific extensions of the current collectors are connected to the external equipment. This means that the site of electrical contact is outside the contour of the cell laminate. This undesirably makes the whole fuel cell stack bulky and lowers the degree of freedom when the fuel cell stack is mounted on equipment.
The object of the present invention is thus to solve the above problems and to provide a polymer electrolyte fuel cell stack that is small in size and light in weight and has a high degree of freedom when the fuel cell stack is mounted on a variety of equipment.